mm: mark async iocb read as NOWAIT once some data has been copied

Once we've copied some data for an iocb that is marked with IOCB_WAITQ,
we should no longer attempt to async lock a new page. Instead make sure
we return the copied amount, and let the caller retry, instead of
returning -EIOCBQUEUED for a new page.

This should only be possible with read-ahead disabled on the below
device, and multiple threads racing on the same file. Haven't been able
to reproduce on anything else.

Cc: stable@vger.kernel.org # v5.9
Fixes: 1a0a7853 ("mm: support async buffered reads in generic_file_buffered_read()")
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/filemap.c

@@ -2199,6 +2199,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
 	offset = *ppos & ~PAGE_MASK;
 
+	/*
+	 * If we've already successfully copied some data, then we
+	 * can no longer safely return -EIOCBQUEUED. Hence mark
+	 * an async read NOWAIT at that point.
+	 */
+	if (written && (iocb->ki_flags & IOCB_WAITQ))
+		iocb->ki_flags |= IOCB_NOWAIT;
+
 	for (;;) {
 		struct page *page;
 		pgoff_t end_index;